Vacumn at WOT?
 

Another question from a long time FI guy. I'm used to a good fuel injection manifold having almost no engine vacumn at WOT. If not, your throttle is too small.

I realize that with a carb, it's always a compromise, but I'm wondering what is considered "good" for a high performance application where Idle is still important? I'm getting just under 1 in/hg at WOT (from a calibrated sensor). I figure that's about a 15-20hp loss. Is that about the best I can expect?

not sure about the logic... your premise is essentially correct. i.e. if the annular opening is large enough there should be near zero vaccum. what i am not quite getting is your suggestion that , somehow this costs you power.... if the motor pulls a 1 atm "vacuum" on the inlet stroke and the atmosphere is 1 atm, then the motor gets all the air that a normally aspirated motor will... so you get what you get and make the power you are going to make with it... in theory... obviously if its a carb it needs sufficient signal to actually function like a carb but making that assumption, i.e. that the carb does actually work,
where would you lose power ?

never had vacumn at wfo, just boost:drink:

Haha! That's exactly what my buddy said :)

Steve: A two barrel with 5in/hg depression at WOT would provide an EXCELLENT signal to the carb, but would be down 20-30% in power output vs. a good 4 barrel. A fuel injection system needs no vacumn signal going through the carb and hence can big big enough that there is almost no depression in the manifold at WOT. In a perfect world, a perfect carb with a perfect venturi would also recover 100% of the pressure loss accross the carb at WOT.

What I am wondering is what is considered reasonable. At some point, running a bigger carb would have very little difference on the pressure loss and would only show reduced part throttle performance

of course. but that wasn't your premise... you said "or your throttle is too small" which is of course, correct... and again which, with a 2 barrel, it is. the answer to this question is simple. dennis moore answers it on page 147 of his book. it gives the arithmatic for calculating precisely the carb you need for a given displacement at given revs. anything smaller than that and you leave something on the table. anything bigger doesn't help you.

Those calculations are anything but precise. In fact if optimizing your combination is the goal, they are more like gross generalizations

If your looking for great performance be careful with those Holley calculators and suggestions. They tend to be very conservative.

For someone that wants to just bolt a carb on and go usually anywhere from 1.0"Hg-1.5"Hg will work pretty well and give decent performance.

For those that can tune carbs or have a good carb tuner than many will shoot for .7"Hg-.9"Hg. Some people will go lower, but they need to really understand how to tune the carb and ignition. Tune does not mean just jets and PV's.

BTW: When testing this way test with flame arrestor on and off. You may find your arrestor is choking you.